Supramolecular isomerism, single-crystal to single-crystal transformation induced by release of in situ generated I2 between two supramolecular frameworks

Article abstract of DOI:10.1039/c3dt50183c

Two supramolecular isomers, [H₂tpim·H₄betc] ·I (1) and [H₂tpim·H₄betc]·I ₃·2H₂O (2), have been synthesized by one-pot reactions. Single-crystal to single-crystal transformations are observe

Full text of DOI:10.1039/c3dt50183c

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Supramolecular isomerism, single-crystal to single-
crystal transformation induced by release of in situ
generated I2 between two supramolecular
frameworks†
Cite this: Dalton Trans., 2013, 42, 5619
Received 18th January 2013,
Accepted 4th March 2013
Xing Meng,a,b Xue-Zhi Song,a,b Shu-Yan Song,a Sheng-Qun Su,a,b Min Zhu,a,b
Zhao-Min Hao,a,b Shu-Na Zhaoa,b and Hong-Jie Zhang*a
DOI: 10.1039/c3dt50183c
www.rsc.org/dalton
Two supramolecular isomers, [H2tpim·H4betc]·I (1) and obtained from the organic linker with a given metal ion under
[H2tpim·H4betc]·I3·2H2O (2), have been synthesized by one-pot the same conditions may belong to various networks.6b For
reactions. Single-crystal to single-crystal transformations are example, Matzger and co-workers have reported the co-exist-
observed between the two supramolecular isomers which are ence of heterogeneous phases based on two kinds of carboxy-
induced by release of in situ generated I2.
late linkers, showing that the ratios of linkers play an
extremely important role in the formation of these phases.7
Promoted by the pioneering achievements, we chose another
strategy to synthesize coordination polymers. The additives,
such as acids, amines and so on, have been always employed
in the field of crystal engineering.8 However, in the presence of
the same additive, the case where different phases generate
from the solution is lacking under the same experimental con-
ditions. Based on this consideration, phosphoric acid (H3PO4)
was selected as the additive to construct coordination poly-
mers. In this communication, it is clear that H3PO4 plays an
important role in directing the formation of one phase over
others in the process of crystallization. As illustrated in Fig. 1,
different phases are generated by a hydrothermal reaction with
ZnI2·4H2O, 2,4,5-tri(4-pyridyl)-imidazole (Htpim) and benzene-
1,2,4,5-tetracarboxylic acid (H4betc) in the presence of H3PO4
with different volumes at 140 °C for 3 days. One amorphous
powder and two distinct crystalline phases are observed as the
volume of H3PO4 increases. When the volume of H3PO4 is
lower than 0.1 mL, an unknown grey powder was the only
product. Block crystals of 1 formed together with the unknown
powder at a slightly higher volume of H3PO4 (0.1 mL < volume
Design and synthesis of supramolecular isomers has received
more and more interest since the concept ‘supramolecular iso-
merism’ was discussed by Zaworotko in 2001.1 Supramolecular
isomerism includes polymorphism and pseudopolymorphism,
which has been used to describe the existence of various types
of supramolecular synthons resulting from the same reac-
tants.1a,2 In some cases, supramolecular isomerism could be
in agreement with polymorphism. However, in others, supra-
molecular isomerism is not true polymorphism, because
different guest molecules encapsulated in the lattice lead to
distinct chemical compositions. As far as we know, the for-
mation of guest-dependent frameworks has been well docu-
mented.3 If the guest molecules can be released or removed,
single-crystal to single-crystal transformations may occur
among these crystals.4 Nevertheless, guest-induced supramole-
cular isomerism coupled with single-crystal to single-crystal
transformations has been rarely explored.
In the past decade, the hydro/solvo-thermal syntheses have
been rapidly developed to obtain crystals.5 But an elaborate
relationship between the synthetic conditions and architec-
tures of the resultant phase from the solution has not been
well established, due to subtle differences of experimental con-
ditions resulting in various structures.6 The architecture
aState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun
130022, P. R. China. E-mail: hongjie@ciac.jl.cn; Fax: +86-431-85698041;
Tel: +86-431-85262127
bGraduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China
†Electronic supplementary information (ESI) available: Additional figures and
tables, PXRD, TGA and the luminescent properties of all compounds. CCDC
913527 and 913528. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt50183c
Fig. 1 Hydrothermal reaction of ZnI2·4H2O, Htpim and H4betc at 140 °C for
3 days yields three different phases depending on the volume of H3PO4.
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giving birth to unit B (Fig. S3a†). The dimer is overlaid
together through C–H⋯π contacts (a distance of ca. 3.415 Å)
between the pyridyl rings of H2tpim+ and intermolecular
hydrogen bond interaction (N2–H⋯O2 with a distance of ca.
2.804 Å; C4–H⋯O2 with a distance of ca. 3.227 Å; C1–H⋯O3
with a distance of ca. 3.228 Å), and an infinite supramolecular
chain is formed (Fig. S3b†). These adjacent 1D chains are
mutually interlinked via π⋯π interactions (a face to face dis-
tance of ca. 3.456 Å) between pyridyl rings of H2tpim+ and
extend to a 2D layered sheet (Fig. S3c†). These 2D layers are
arrayed orderly via a hydrogen bond between an uncoordi-
nated oxygen atom (O(7)) of carboxylate groups of the H4betc
ligand and a nitrogen atom (N(5)) of the pyridyl ring of
H2tpim+ with an O7⋯N5 distance of 2.656 Å and then extend
to a 3D supramolecular framework with positive charges
Fig. 2 Ball-and-stick representation of compounds 1 (a) and 2 (b). Color code:
O, red; C, grey; N, blue; I, purple; H, white. Some hydrogen atoms have been
omitted for clarity.
< 0.2 mL). The formation of the grey powder decreases with
the increasing amount of H3PO4, while the amount of 1
increases. When the volume of H3PO4 is greater than 0.2 mL,
the white powder disappeared and new needle-shaped crystals
of 2 formed along with 1. When the volume increases to
0.5 mL, 2 is the single phase resulting from the solution.
Co-crystals of 1 contain one isolated protonated H2tpim+
segment, one H4betc and an I− anion, as shown in Fig. 2a. It
crystallizes in the monoclinic space group C2/c with Z = 8 in
the asymmetric unit.‡
−
(Fig. S3e†). In addition to the I3 fragment, an electrically
neutral architecture is formed (Fig. S3f†).
To sum up, two supramolecular isomers have been syn-
thesized by adjusting the amount of H3PO4. It deserves to be
mentioned that H3PO4 are identified as key reagents in the for-
mation of two compounds. In the reaction system, H3PO4 plays
three important roles: (i) Htpim was protonized by H3PO4; (ii)
at high temperature, I− anions were easily oxidized by H3PO4,
generating I2; (iii) it creates an acidic environment, inhibiting
the deprotonation of H4betc, so Zn ions cannot take part in
the coordination. Compounds 1 and 2 are obtained in turn.
The results indicate that the concentration of H3PO4 plays an
important role in the process of crystallization. An example
presented here can also demonstrate this hypothesis: when
the volume of H3PO4 is 0.1 mL, the mole ratio of H3PO4–
Htpim–I− is 17 : 3 : 10, the amount of H3PO4 is larger than the
sum of Htpim and I−, but crystals of 2 are not found. The poss-
ible mechanism is listed below: Htpim has been firstly proto-
nized by H3PO4 at low temperature, then the oxidation of I−
anions occurs at high temperature and high concentration due
to the low oxidizability of H3PO4. When the concentration of
H3PO4 is low, only I− anions balance the charge of the supra-
molecular framework constructed by H4betc and protonated
H2tpim+, generating 1. When the concentration of H3PO4
reaches a high level, I− anions begin to oxidize, and I3− anions
are generated, coexisting with I− anions. So compounds 1 and
2 are formed together. The fact implies that the formation of
different isomers might be attributed to the concentration of
H3PO4. Controlled experiments have been also carried out:
when H3PO4 is replaced with HCl, only grey powders and crys-
tals 1 are obtained. The experimental results indicated that the
oxidizability of the acid used plays a key role in the formation
of 2.
In order to understand the supramolecular arrangement of
the network, the π⋯π and hydrogen bonding interactions
(Table S1†) between the molecules are analyzed in detail. Two
H2tpim+ molecules aggregate together through π⋯π contacts
between the pyridyl rings (a face to face distance of ca. 3.445 Å)
giving birth to a cationic dimer due to its protonation
(Fig. S2a†). Two H4betc molecules are bonded to the dimer
through a series of hydrogen bonds (C(10)–H(10)⋯O(7) with a
distance of ca. 3.137 Å; N(3)–H(3)⋯O(8) with a distance of ca.
2.769 Å; C(2)–H(2B)⋯O(8) with a distance of ca. 3.106 Å) gene-
rating unit A (Fig. S2b†), which then further connects through
C(16)–H(16)⋯O(6) (a distance of ca. 3.002 Å) and O(1)–H(1A)
⋯N(5) (a distance of ca. 2.733 Å) hydrogen bonding inter-
actions, generating an infinite supramolecular chain with a
wave-like shape (Fig. S2c†). The adjacent chains are mutually
interlinked via the hydrogen bonds between the carbon atom
of H2tpim+ (C(17)) and the uncoordinated oxygen atom (O(3))
of carboxylate groups of the H4betc ligand with a C17⋯O3 dis-
tance of 3.190 Å and extend to a 2D layered sheet with positive
charges (Fig. S2d†). Naturally, the I− ion stands on the surface
of each layer to compensate for it, and electrically parallel
neutral sheets are formed. These 2D layers are arrayed orderly
via electrostatic force along the b-axis and then extend to a 3D
neutral supramolecular framework (Fig. S2e†).
Co-crystals of 2 contain one isolated H2tpim+ cationic
−
segment, one H4betc, an I3 anion and two water molecules,
as illustrated in Fig. 2b. Single-crystal X-ray diffraction analysis
reveals that compound 2 crystallizes in the triclinic space
It should be mentioned that the available void provides the
opportunity for release of the I2 molecule in 2. The effective
free volume of 29% is calculated by PLATON.9 It should be
noted that the iodine content is high with about 39.2% (wt%),
which is slightly lower than the reported value (43.2%) for
ˉ
group P1 with Z = 2 in the asymmetric unit.
The most interesting and remarkable feature of the con-
struction of the 3D supramolecular architectures in 2 is the
hydrogen bonding (Table S2†) and π⋯π contacts. One H2tpim+
and one H4betc aggregate together through intermolecular
hydrogen bonds (O6–H⋯N4 with a distance of ca. 2.644 Å)
coordination polymer {[Cu6(pybz)8(OH)2]·I5−·I7−
} (pybz = 4-
n
pyridylbenzoate).10 Furthermore, it represents the first
example that I2 molecules are generated in situ from the
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Fig. 3 An interesting SCSC transformation occurs between 2 (a) and 1 (b)
Fig. 4 (a) Temporal evolution of UV/Vis absorption spectra for the I2 delivery
from 5 mg of compound 2 in 50 mL methanol solution and (b) the delivery of I2
in the first 840 min.
through release of I2.
reaction system, differing from the reported ones, in which I2
molecules are introduced at the beginning.10,11
release of I2. Furthermore, the delivery of I2 can be influenced
by the solvent and various anions. In addition, the lumine-
scent properties of all compounds have also been investigated.
The authors are grateful to the financial aid from the
National Natural Science Foundation of China (Grant Nos
21001101, 21001100, 91122030 and 21210001), ‘863’-National
High Technology Research and Development Program of
China (Grant No. 2011AA03A407) and National Natural Science
Foundation for Creative Research Group (Grant No. 21221061).
Prompted by the achievements about controlled release of
iodine molecules in organic solvents, we attempted to release
iodine molecules from the network of 2. When brown crystals
of 2 were soaked in dry methanol, they undergo a naked-eye
detectable change in color. As illustrated in Fig. S4,† the color
of methanol solution intensified gradually from colorless to
yellow, and the color of the crystals of 2 changed from brown
to pale yellow. The X-ray diffraction pattern confirmed that the
phase transformation was complete and that the bulk product
2′ was identical to 1 (Fig. S5†). Fortunately, an interesting SCSC
transformation is achieved through this iodine release process
(Fig. 3).
Notes and references
In order to investigate the kinetics of I2 release of 2 in
crystal transformation, the in situ UV/Vis spectrum was
measured at room temperature. By simply immersing 5 mg of
crystals in 50 mL methanol solution, the absorbance of I2 in
methanol solution increased with time, and the release of
iodine became slower later because the concentration of I2 in
‡Synthesis of compounds 1 and 2: ZnI2·4H2O (0.5 mmol), 2,4,5-tri(4-pyridyl)-
imidazole (0.3 mmol), benzene-1,2,4,5-tetracarboxylic acid (0.5 mmol), H3PO4
(0.3 mL) and H2O (10 mL) were sealed in a 15 mL Teflon-lined stainless-steel
autoclave under autogenous pressure and heated at constant 140 °C for 3 days
and then were cooled to room temperature slowly. The resulting block crystals
were collected. Elemental analysis for C28H20IN5O8 (1) (681.39) (%): calcd C
49.43, H 2.81, N 18.65; found C 49.52, H 2.87, N 18.43. Elemental analysis for
C28H24I3N5O10 (2) (971.22) (%): calcd C 34.63, H 2.49, N 7.21; found C 34.52, H
2.56, N 7.33. Pale-yellow block-like crystals of 1 and needle-shaped crystals of 2
were obtained in 32% and 21% yields based on Htpim respectively.
−
methanol increased with time, as shown in Fig. 4. Due to I3
existence in the host framework, we investigated how Cl− ions
influenced the release progress of I2. The UV/Vis spectra of 2
showed that the release rate of I2 increased gradually following
the amount of Cl− ions from 0.2 to 0.8 mmol L−1 (Fig. S7†).
Cl− ions entered into the space of 2 through diffusion, and
interact with I3− to facilitate the release of I2. The influences of
other anions (Br− and NO2−) for I2 release were also investi-
gated. When 5 mg crystals of 2 were soaked in 50 mL methanol
solution containing various ions with the same amount
(0.2 mmol L−1 NaCl, NaBr or NaNO2), the release rate
increased in turn (Fig. S8†). This phenomenon might be
caused by the size effect of anions,12 rather than a stronger
interaction between the cationic framework and the anion.13
In addition, the I2 release in water solution has also been
investigated. Because of the solvent effect, the absorption
peaks had subtle differences (Fig. S9†). The release rate in
water was slower than that in methanol, which might be due
to the fact that I2 is less soluble in water than in methanol.
With the addition of modest amounts of NaCl, the release rate
of I2 increased suggesting that NaCl also accelerated I2 release
in water (Fig. S10†).
Crystal data for 1: C28H20IN5O8, Mr = 681.39, monoclinic, space group C2/c, a =
20.546(5) Å, b = 18.782(5) Å, c = 13.957(5) Å, α = 90, β = 100.677(5), γ = 90, V =
5293(3) Å3, Z = 8, ρcalcd = 1.710 g cm−3, final R1 = 0.0337 and wR2 = 0.0849 (Rint
=
0.0263) for 4685 independent reflections [I > 2σ(I)]. CCDC 913527.
ˉ
Crystal data for 2: C28H24I3N5O10, Mr = 971.22, triclinic, space group P1, a =
7.272(5) Å, b = 12.781(5) Å, c = 18.105(5) Å, α = 78.165(5), β = 83.855(5), γ = 75.892(5),
V = 1594.4(13) Å3, Z = 2, ρcalcd = 2.023 g cm−3, final R1 = 0.0441 and wR2 = 0.0910
(Rint = 0.0208) for 5632 independent reflections [I > 2σ(I)]. CCDC 913528.
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In summary, two supramolecular isomers have been syn-
thesized by one-pot reactions, and single-crystal to single-
crystal transformations are observed which are induced by the
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